London's Riverbank Arena will see action today with semi-finals kicking off in Paralympic football. Generally, football fans can be a rowdy a lot and that's something you've got to think about when building a football venue. Imagine the 80,000 people that fit into a stadium like Wembley jumping up and down in unison to We will rock you. Any structure, no matter how solid, has its own natural frequency at which it likes to vibrate, and if that happens to be in tune with the people's, you can end up with a dangerous sway and ultimate disaster. And that's just one of the things you need to take into account when building a stadium. Find out more in our interview with the engineer Paul Shepherd, who's been involved in building the Arsenal football stadium, in our podcast Stadium maths (the interview with Shepherd starts at 8 minutes 10 seconds).